Molten salt baths for nitriding steel and cast iron



IN MICRONS THICKNESS OF COMPOUND LAYER y 7, J. MULLER 3,317,357

MOLTEN SALT BATHS FOR NITRIDING STEEL AND CAST IRON Filed March 15, 1965 DEPENDENCE OF THICKNESS OF COMPOUND ZONE AND ITS QUALITY ON THE POTASSIUM CONTENT OF THE BATH.

I5 t COMPOUND zone IS 4 COMPOUND ZONE MAY as NON-POROUS AND POROUS AND REMOVABLE O ABLE j: UNDER CERTAIN CONDITIONS OPTIMUM o'lo'z'o'a'o'4o'so'e'o 1'0 Do s'o'lDo POTASSIUM SODIUM CONTENT OF THE BATH CYANATES AND CYANIDES INVENTOR JOHANNES MULLER BY @DMAM =5 ATTOR NEYS United States Patent Ofiice 3,317,357 Patented May 2, 1967 3,317,357 MOLTEN SALT BATHS FOR NITRIDING STEEL AND CAST IRON Johannes Miiller, Neu Isenhurg, Germany, assignor to Kolene Corporation, Detroit, Mich. Filed Mar. 15, 1965, Ser. No. 445,299 3 Claims. (Cl. 148-155) This application relates to molten alkali salt bath processes for nitriding of steel and cast iron. In general, the process hereof represents an improvement over Well known process, such as the process described in a prior Patent No. 3,022,204 of Feb. 20, 1962.

According to the present invention, a molten salt bath is employed, Where, in relation to the pure metal, the potassium content is about 50 to 65% and the sodium content between 35 and 50%. With such a bath com position optimum results are obtained. A non-porous and firm, i.e., substantially non-removable, compound layer is achieved, even under undesirable conditions, as for example, where cast iron parts are being treated or Where the temperature stays below 550 C. for a longer time than is desirable, a time delay which not always can be avoided.

The attached diagram, comprising the last page hereof, illustrates results obtained by the use of a bath of the invention. The compound layer thickness is plotted in relation to the potassium-sodium-metal proportion of the salt melt which contains cyanides and cyanates. From this diagram it will be seen, that up to a content of 65 potassium and 34% sodium, the compound layer thickness is subject only to a relatively slight change. However, where the potassium content is below 50% and the sodium content above 50%, the compound layer obtained might be porous or removable under the above described unfavorable conditions, as for example where cast iron parts are treated, or where the temperature stays below 570 C. for an unreasonable length of time.

On the other hand, Where the potassium content is above 50%, non-porous and firm compound layers have been achieved even when the operating conditions were not of the best.

The diagram shows that, both as to thickness of the compound layer as well as its quality, i.e., non-porosity and non-removability, optimum results are obtained where the K content is 50-65%; and the Na content be tween 35-50%, in the center part of the curve.

The conclusion, as evidenced by the diagram, is that within the range of -50% potassium and 50% sodium extending up to 65 potassium and down to 35% sodium-the nitriding bath hereof evidenced an outstanding margin of safe operation from the point of view of obtaining firm and non-porous and desirably thick compound layers despite unfavorable operating conditions, such as reduced temperature for a long time, or the use of cast iron as the workpieces.

For the initial makeup of such a bath, the following salt mixture was used: 38-44% sodium cyanide; 43-47% potassium cyanate; 20% sodium carbonate; 10-14% potassium carbonate.

During the operation of the bath, replenishment is required. A desirable replenishment mixture which has been used is preferably 44-59% potassium cyanide, and 41-56% sodium cyanide.

I point out that it would be advantageous to employ for the bath, an iron free pot, as for example, a pot having a titanium lining. I also point out that the bath is improved in its operation by aeration as described in said patent, namely, it would be advisable to aerate the salt bath in accordance with said patent, that is to say, to pass air or other oxygen containing gases, in a fine distribution upwardly through the deep pot containing the bath, all according to said patent. Hence, both the use of a titanium pot and aeration are here included.

Now having described the process hereof, reference should be had to the claims which follow.

What is claimed is:

1. A process of nitriding steel and cast iron comprising immersing a ferrous metal workpiece in a molten alkali salt bath comprising mainly cyanides and cyanates of alkali metals of the group consisting of potassium and sodium; characterized by the KNa content, based on the pure metal content, ranges from 5065% K, and 35- 50% Na.

2. A process according to claim 1 wherein the initial makeup of the bath consisting of 38-42% NaCn; 43-47% KCNO; plus carbonates in an amount of 26% Na CO and 10-14% K CO 3. A process according to claim 1 wherein replenishment of the bath is effected by adding a replenishment mixture consisting of 44-59% KCN and 41-56% NaCN.

References Cited by the Examiner UNITED STATES PATENTS 1,920,368 8/1933 Egan 148-155 X 2,927,875 3/1960 Muller 148-l5.5 3,022,204 2/1962 Muller et al 148-155 FOREIGN PATENTS 1,067,835 10/ 1959 Germany.

DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner. 

1. A PROCESS OF NITRIDING STEEL AND CST IRON COMPRISING IMMERSING A FERROUS METAL WORKPIECE IN A MOLTEN ALKALI SALT BATH COMPRISING MAINLY CYANIDES AND CYANATES OF ALKALI METALS OF THE GROUP CONSISTING OF POTASSIUM AND SODIUM; CHARACTERIZED BY THE K-NA CONTENT, BASED ON 